1. Field of the Invention
This invention relates to a rotating electric machine having a stationary field coil wound and disposed coaxially around the rotational axis thereof; in particular, it relates to such a rotating electric machine utilized as a starter-generator of an automotive vehicle.
2. Description of the Prior Art
In general, a DC motor and a separate synchronous AC generator are used as the engine starter and the charging generator, respectively, of an automobile. However, for the purpose of achieving maximum compactness and economy, a single rotating electric machine is sometimes utilized both as the engine starter and the charging generator of an automobile. Japanese Patent Publication No. 61-54949, for example, disclose a type of such rotating electric machine. Although synchronous rotating electric machines usually comprise a field winding mounted on the rotor and an armature winding disposed on the stator, the starter-generator disclosed in the above-mentioned Japanese patent comprises a stationary field winding wound and disposed coaxially around the rotational axis of the machine, as described below.
FIG. 1 shows an upper half portion of an axial section of a starter-generator of the type disclosed in the above-cited Japanese patent. The starter-generator, directly attached to the internal combustion engine situated on the left side in the figure, comprises a rotor 1, a field winding 2, and an armature 3. The rotor 1 comprises a pair of field pole forming members 1a and 1b both made of a ferromagnetic material. The substantially disk-shaped pole forming member 1a, which also acts as a fly wheel of the engine, is fixedly secured to the end of the crank shaft 4 of the engine by means of bolts 5, and peripherally comprises a comb-shaped structure having circumferentially spaced teeth or projections extending in the axial direction toward the member 1b on the left side in the figure. On the other hand, the pole forming member 1b consists of a comb-shaped structure having circumferentially spaced teeth or projections extending in the axial direction toward the member 1a. These projections of the field pole forming memebers 1a and 1b extending in opposite axial directions alternate each other in the circumferential direction to form circumferentially spaced alternating north and south field poles along the circumference of the rotor 1. The field winding 2 for exciting field pole forming members 1a and 1b consists of a coil wound on a field core 6 around the central rotational axis C of the rotor 1. The field core 6 is secured to a rear plate 7, opposing the field pole forming members 1a and 1b with a small radial gap, respectively. The armature 3 comprises armature cores 3 a which are mounted to and extending radially inwardly from the housing 8 to oppose the circumferential surface of the rotor 1 with a small radial gap; the armature 3 further comprises an armature winding consisting of coils 3b wound around the cores 3a and connected in three-phase Y-connection. Further, the field pole forming member 1a is coupled to a clutch 9 which connects and disconnects the rotational force between the crank shaft 4 and the driving shaft 10 of a reduction gear (not shown) at the right side in the figure. The diaphragm spring clutch 9 comprises clutch disk 11 secured to the driving shaft 10, pressure plate 12, a disk-shaped diaphram spring 13 for urging the plate 12, and a clutch cover 14 secured to the field pole forming member 1a by means of bolts 15. Crank angle detector 16, on the other hand, detects the rotational angle of the chrank shaft 4, or more precisely, rotational angles of the field poles of the rotor 1.
The operation of the starter-generator device of FIG. 1 is roughly as follows. When the device acts as a starting motor, field winding 2 is supplied with a DC current to excite and form alternating field poles along the circumference of the rotor 1. At the same time, armature winding 3b is supplied with an AC current to form a rotating magnetic field. Thus, magnetic flux .PHI. is formed through the field core 6, field pole forming member 1a, armature cores 3a, and field pole forming member 1b, and the resulting torque drives and rotates the rotor 1 of the device. On the other hand, when the device acts as an AC generator, field winding 2 is supplied with a DC current and the rotor 1 is driven and rotated by the engine; the AC voltages induced in the armature winding 3b by rotating field poles formed on the rotor 1 are rectified into a DC voltage to charge the battery, etc.
The conventional starter-generator as described above, however, has the following disadvantage. Since the field coil 2 is disposed coaxially with the rotational axis C and a DC current flows therethrough, a leakage flux .PHI.' shown by dotted arrows is inevitably formed, in addition to the normal flux .PHI.. This leakage flux .PHI.' runs around rotor 1 and armature 3, and, near the rotational axis C, passes through the crank shaft 4 of the engine and the driving shaft 10 of the reduction gear. As a result, the crank shaft 4 and the driving shaft 10 are magnetized by the leakage flux .PHI.'. When oil is used as the lubricant, particles of magnetic materials, which are formed by the abrasion of these shafts, gears, etc., and floating in the lubricant oil, are also magnetized and adhere not only to these shafts 4 and 10 but also to bearings and gears coupled thereto. This promotes further abrasion of these shafts, bearings, and gears, to reduce the durability and reliability of the machine.